Add initial Sign In structure

a. Add sessions controller, view (new.html.erb) and basic specs
b. Added sessions as resource to routes, configured signin & signout paths
Finish 8.2.1: Create user remember token

1. Add database column to users table to store remember token
2. Created code in user model to generate secure token
Finish Sxn 8.2.4: Basic signin mechanics

1. Completed signin mechanics in sessions helper
2. Added logic to change header menu if user signed in
Finish Sxn 8.2:  Completed Sign-in/Sign-out Machinery

1. Redirected new users to profile page after signing up
2. Created test and code to destroy session upon signout, redirect to root